In a point to multi-point wireless communication system, a number of radio channels provide a connection between remote (mobile) access or field units and a central location, such as a base station or access point. In a Code Division Multiple Access (CDMA) system, a number of different channels can be transmitted on a single radio frequency carrier by applying different codes to each signal. However, even in a CDMA system, demand for access to channels is so great that the base station must allocate and switch the channels among multiple users.
CDMA type multiple access schemes are generally thought to, in theory, provide the most efficient use of the radio spectrum. CDMA schemes only work well, however, when the power levels and transmit times of individual transmissions are carefully controlled. The original widely deployed CDMA voice wireless systems such as IS 95B use two different types of power control on the uplink in order to ensure that a signal from a given field unit arriving at the base station does not interfere in a disruptive manner with the signals arriving from other field units. In a first process, referred to as open loop power control, a rough estimate of the proper power control level is established by the mobile field unit itself. In particular, after a call is established and as the mobile moves around within a cell, the path loss between the field unit and the base station will continue to change. The mobile continues to monitor the receive power and adjust its transmit power. The mobile measures a power level on the forward link signal as received from the base station and then sets its reverse link power accordingly. Thus, for example, if the receive power level is relatively weak, then the mobile assumes that it is relatively distant from the base station and increases its power level. The converse is true, in that a signal received at a relatively high level indicates that the mobile is relatively close to the base station and therefore should be transmitting with reduced power.
Since the forward and reverse links are on different frequencies, however, open loop power control is inadequate and too slow to compensate for fast Rayleigh fading. In other words, since Rayleigh fading is frequency dependent, open loop power control alone cannot compensate for it completely in CDMA systems. As a result, closed loop power control is also used to compensate for power fluctuations. In the closed loop process, once the remote unit obtains access to a traffic channel and begins to communicate with the base station, the base station continuously monitors the received power level on the reverse link. If the link quality begins deteriorating, the base station sends a command to the mobile via the forward link to increase its power level. If the link quality indicates excess power on the reverse link, the base station commands the mobile unit to power down. This is typically done by having the base station send power control commands to the mobile using a specially encoded message sent on a forward link traffic channel.
Unlike voice traffic, the user of a wireless data service may be switched on, but not actively sending or receiving data. Accordingly, wireless data users may be in an “active” mode (currently allocated a wireless data traffic channel for sending or receiving data), an “idle” mode (operational, but not currently sending or receiving data) or “off” (not communicating at all). An idle user may, for example, have just sent or received a data traffic transmission and is therefore deemed likely to soon request a data traffic channel for further transmissions. A maintenance message may therefore be employed to maintain a user in a synchronized, but idle, state to facilitate allocation of a wireless traffic channel when needed. When a user requests a channel, the idle state allows the user to be allocated a wireless traffic channel more quickly than for a user who was not being maintained in a synchronized idle state. For more information concerning one way to implement a system for quickly switching states, please refer to U.S. Pat. No. 6,222,832, entitled “Fast Acquisition of Traffic Channels for a Highly Variable Data Rate Reverse Link of a CDMA Wireless Communication System@ assigned to Tantivy Communications, Inc., assignee of the present application.
A number of users, therefore, may be maintained in an idle state through a periodic sequence of maintenance messages. In the idle state, the maintenance messages typically provide time tracking and power control. These maintenance messages, however, are typically sent at a similar power level during both the idle and active states so that the base station can be assured of receiving them. Unfortunately, the maintenance messages can increase interference with other active units since they are still transmitting energy.